Electron microscopy has been highly developed, and high-resolution electron microscopy has enabled observation of individual atoms. Today, commercial instruments are required to satisfy many demands, such as new functions, easy operations, and high cost performance. Therefore, microprocessors have been incorporated in electron microscopes. This permits storage of specimen positions, automatic montaging, image rotation, and display of the present conditions of the microscope, which could not be envisaged before the introduction of microprocessors. Accurate beam alignment, accurate correction of astigmatism, and optimum focusing are crucial for practical use of a high-resolution electron microscope. If a computer performs these jobs instead of the operator, a tremendous amount of labor and time would be saved, and any operator can make the best use of the microscope. Besides these demands for control over the microscope, many other requirements, such as image simulation and image processing, are imposed on high-resolution electron microscopy. However, the present microprocessor is not capable of meeting all of these demands.
Recently, external computer and an image frame memory have been combined with an electron microscope, as disclosed in an article entitled "An Electron Microscope Controlled by an External Computer" by Y. Kokubo, K. Suzuki, S. Mori, J. Suzumi, M. Taira, and A. J. Skarnulis, in Proc. XIth International Congress on Electron Microscopy, Kyoto, 1986, pp. 497-498.
An electron microscope, such as a transmission electron microscope or scanning electron microscope, includes an electron gun, a high voltage-generating circuit, electromagnetic lenses, deflector coils, power supplies for the electromagnetic lenses, and power supplies for the deflector coils. The number of the electromagnetic lenses tends to increase whenever a new function is added to the microscope. The latest electron microscopes each have 9 electromagnetic lenses. Also, the number of deflector coils increases in proportion to the number of the electromagnetic lenses.
As mentioned previously, a recent electron microscope incorporates a computer such as a microprocessor. This microscope also includes a read-only memory (ROM) in which data about the optimum values of electric currents supplied to the electron lenses and the deflector coils is stored. These values of the currents are determined according to preset values of magnification and preset values of accelerating voltage. The combinations of the values of these currents are so set that the image does not rotate if the magnification or the accelerating voltage is changed. When the operator keys in the magnification and the accelerating voltage, the internal computer controls the electron gun power supply, the electron lens power supply, and the deflector coil power supply, based on the data stored in the ROM. The data stored in the ROM indicates discrete values of the magnification and discrete values of the accelerating voltage.
An attempt has been made to analyze the image signal obtained by such an electron microscope incorporating a computer, by means of an external computer connected to the microscope. In the prior art combination of the electron microscope and the external computer, the external device produces signals only to independently energize the electromagnetic lenses and the deflector coils of the microscope. Therefore, if the currents fed to the lenses and coils are varied to change the magnification, then the image will be rotated. In extreme cases, the currents may increase or decrease intolerably. This makes it impossible to observe the image, or the microscope breaks down. Consequently, when the operator desires to change the magnification or the accelerating voltage under the instruction of the external computer, the only one possible method is to select the closest value of the magnification or voltage from the values stored in the ROM. Thus, it is impossible to energize the electromagnetic lenses and the deflector coils so as to achieve desired magnification and accelerating voltage without causing rotation of the image or other undesirable situation. Similar problems arise in varying the size of the electron beam spot, the amount of defocusing of the objective lens, or the amount of correction made for astigmatism.